Lithium ion batteries, lithium ion capacitors, electric double-layer capacitors and the like require a higher voltage to increase their energy density. To increase the energy density, it is preferable to reduce the potential of the negative electrode by using pre-doping techniques. For efficient pre-doping, the current collector needs to have through holes. Lithium ions can then move reversibly through the through holes of the current collector, so that the lithium ions can be intercalated in the negative electrode active materials.
Known methods of fabricating current collectors having through holes include, for example, punching, meshing, expanding, and netting techniques. The size of the through holes formed by these methods is typically 0.3 mm or more. However, forming through holes accordingly lowers the strength of the current collector, and this issue of reduced strength is more significant with such a relatively large hole diameter.
To deal with this, electrodes or the like using current collectors having relatively fine through holes have been proposed. For example, a lithium ion capacitor is known, which includes a positive electrode made of a material capable of reversibly intercalating lithium ions and/or anions, a negative electrode made of a material capable of reversibly intercalating lithium ions, and electrolytic made of a solution of lithium salt in a non-protonic organic solvent, wherein (1) the negative and/or positive electrodes are doped with lithium ions by an electrochemical contact between the positive and/or negative electrodes and a lithium ion source; (2) the positive electrode has a potential of 2.0 V or less after short-circuited to the negative electrode; and (3) the positive and/or negative electrodes has/have a current collector made of a metal foil having a plurality of holes extending through the front and the back sides, these through holes having an average diameter of inscribed circles of 100 μm or less (Patent Document 1).
Also known is a coated electrode including an electric collector made of an aluminium foil having a plurality of through holes extending through the front and back sides with a thickness of 20 to 45 μm, an apparent density of 2.00 to 2.54 g/cm3, and an air permeability of 20 to 120 s, and an electrode layer formed by applying a coating on this electric collector, the coating containing a material capable of reversibly intercalating lithium ions and anions as an active material, wherein 80% or more of the through holes of the electric collector have a hole diameter of 1 to 30 μm (Patent Document 2). Aluminium foils for electrolytic capacitors having aligned crystal orientation are also known (for example, Patent Documents 3 and 4).
However, if formed with a plurality of through holes, aluminium foils would inevitably have a lower strength. This also applies to the conventional techniques mentioned above, and as a result of reduced foil strength due to the formation of through holes, there is a risk that the foil may break or wrinkle during a subsequent step where active materials are coated on the aluminium foil. Even if the coating were achieved successfully, the aluminium foil in the end product would be prone to break upon an impact applied to the product. The smaller the aluminium foil thickness, the more serious these issues would be.
Meanwhile, batteries and capacitors are highly demanded to be lighter and smaller in addition to the demands for higher energy density and higher output density. With these demands, current collectors are desired to be thinner.    Patent Document 1: Japanese Patent Application Publication No. 2007-141897    Patent Document 2: WO2008/078777    Patent Document 3: Japanese Patent Application Publication No. 2009-62595    Patent Document 4: Japanese Patent Application Publication No. 2005-174949
While formation of a large number of through holes is desired for enhancing the performance as current collectors or the like, this will likely lead to reduction in foil strength, which in turn may cause troubles in subsequent processes. There are methods for increasing the strength of foil subjected to etching such as adding alloying elements (for example Fe, Cu, Mn, Mg, Ti) or adjusting heat treatment conditions, none of which, however, can be called the best since these methods all inhibit etching. Inhibiting etching here refers to etch pits stopped from growing, or excessive dissolution where normal dissolution cannot be maintained. With a conventional high-strength aluminium foil, typically 3003 or the like, it is difficult to control formation of pits such that a large number of pits extend from a front surface to a back surface. The foil could be processed somehow before the etching to provide strength, but this method will inhibit formation or growth of etch pits, which results in poorer balance between strength and air permeability.
Under the circumstances, as described above, aluminium foil that can exhibit desired foil strength despite a large number of through holes is yet to be developed.